The present invention relates generally to disk drives for signal reproducing apparatus and relates more particularly to speed control of a brushless direct current motor utilized for that purpose.
Disk drives are commonly used in signal reproducing apparatus. A disk platter or a plurality of disk platters are utilized for the recording and the reproducing of signals on a magnetic record media. A magnetic disk spins around a spindle supporting the magnetic disk, a transducer is positioned on a record track located on the disk platter which then records and reproduces signals on that record track. The disk platter is typically rotated around its spindle at a relatively constant speed by a spindle drive motor, e.g. 3,125 revolutions per minute. The speed of the transducer with respect to the record track and hence the speed of the spindle drive motor is critical for the accurate and dense recording of signals by the transducer.
In signal reproducing apparatus employing disk drives, it is important to minimize the surge currents required by the disk drive and to minimize the cost of the components utilized. The spindle drive system is a system which has required substantial surge currents while starting from standstill and lesser, but still substantial, surge currents during steady running because of the widespread use of so-called "bang-bang" or "drive-coast" speed control systems. The bang-bang speed control system has a speed variation inferior to that obtained with a continuous speed control system.
Bang-bang speed control systems draw bursts of the full surge current in between "coasting" periods. This results in a speed which oscillates continuously up and down over a range of typically .+-.0.5%. Furthermore, the power supply is continually subjected to surge currents even while the motor is running at nominal speed. These surge currents cause transient voltages in the power supply distribution system. These transient voltages can have a deleterious effect on other disk drives, tape drives, etc. which typically operate on the same power distribution system.
A motor utilized in a spindle drive capacity in a disk drive system may typically be a brushless direct current motor. The system being driven by the spindle motor has a relatively large inertia and is relatively viscous. The system has a relatively large inertia due to the mass of the disk platter or multiple platters utilized. The system is especially viscous due to the turning resistance of the spindle drive motor, the spindle itself and associated grease and bearing drag, and to the drag which air flowing over the platter surface creates.